In optical communication systems using optical fibers for the optical signal transmission paths, when the polarization mode dispersion of the optical fiber is excessively large, deterioration of the pulse waveform of signal light guided by that optical fiber increases resulting in deterioration of transmission capacity. Thus, optical fibers are required to have a low level of polarization mode dispersion.
Patent Document 1 discloses a technology of producing a coated optical fiber in which resin coating is carried out on the surface of a glass optical fiber for use as an optical fiber having low polarization mode dispersion. Namely, in the technology disclosed in Patent Document 1, polarization mode dispersion is reduced by providing a twist to the coated optical fiber by a twist provider and periodically changing the direction of that twist relative to the lengthwise direction. In addition, Patent Document 1 also describes that the amount of twist provided increases the shorter the free path line of the coated optical fiber downstream of the twist provider (hereinafter referred to as a downstream free path line), and that this is preferable in terms of reducing polarization mode dispersion.    Patent Document 1: U.S. Pat. No. 7,317,855